The present invention relates to a rigid reelable heated pipe for transporting fluids such as hydrocarbons, particularly in the submarine environment.
In application associated with offshore transport pipe for hydrocarbons, there is a need for maintaining a temperature level in the pipe during service as well as during production shutdowns and re-starts. Therefore, the majority of pipes are equipped with thermal insulation means that contribute to maintaining an acceptable temperature level. However, in certain cases, the thermal insulation solution is inadequate for maintaining the required temperature level and the pipe must then be heated actively, that is to say with a heat supply which does not come from the transported fluid. This active heating solution for pipes is used in certain specific applications in conjunction with the aforesaid thermal insulation; it is particularly recommended when the pipe is subjected to prolonged periods of production stoppage.
The pipes which are concerned by the present invention are of the rigid type, as opposed to pipes of the flexible type. Laying a pipe or a rigid pipe on the seabed is done most frequently using a laying vessel. The laying is called S laying when the pipe has the shape of an S between the laying vessel and the seabed and it is called J laying when the pipe adopts the shape of a J. In the latter case, a guide ramp or a stem is provided on the laying vessel, which ramp may sometimes be partially immersed in the water. These laying operations are described in the API (American Petroleum Institute) publication Recommended Practice 17A, 1987.
According to these techniques, the rigid pipe to be laid can be stored on the laying vessel in pipe sections of a given, but relatively short length; the pipe sections being connected together as they are laid. According to another solution, so-called rigid unreeled, suitable also for pipe-in-pipe pipes, the pipe is manufactured in long lengths on land and reeled onto a storage reel located on the laying vessel; the pipe is then unreeled from said reel during the laying operation. It is this latter type of rigid pipe, but capable of being wound on large diameter reels, to which this invention relates to. The rigid unreeled solution has certain advantages relative to the J or S laying: the laying operations are faster, since the pipe is already assembled on land, even if, in return, loading of the laying vessel and manufacturing (assembly of the pipe) are substantially longer operations. Furthermore, the pipe manufacturing on land (welding, checking) is done in better conditions.
The reelable rigid pipes of the invention are of two types: single envelope and double-walled envelope, respectively. A single envelope pipe comprises a unique tube used for transport. In order to limit thermal exchange with the outside, a single envelope pipe generally comprises an external insulating covering around the pipe. A double-walled envelope pipe, commonly called a pipe-in-pipe in the petroleum industry, comprises two coaxial pipes, inside and outside, respectively, separated by an annular space. The inner pipe or “flow line” is used to carry the hydrocarbons. The outer pipe or the “carrier pipe” provides an annular space that can be used for different purposes (insulation, heating, monitoring, accommodation of secondary pipes). Thermal insulation can be particularly useful for avoiding cooling of the fluid being transported and the formation of hydrates; hydrate formation being particularly associated with production stoppages; the annular space can also contain passages for various fluids (water, air, heating liquids, etc) as well as electrical cables (for heating or for carrying an electrical signal, etc.). For keeping the integrity of the pipe-in-pipe during the different operations as well as in service, auxiliary devices are used such as annular spacers, centralizers or limiters of propagation of radial deformation, for example.
Active heating systems are known for installation in the annular space of pipe-in-pipes. These systems use wound secondary pipes, for example, enabling the circulation of a heat transfer fluid or using the annular space directly for circulating a heat transfer fluid. Other ones use electrical energy directly (GB 2,084,284) or use heating electrical cables.
However, when the unreeled rigid technique is used, the pipe must be wound in advance onto a reel; the active heating systems using heating cables cannot be used due to the elongation or stretching stresses present on the extrados of the pipe and the compression present on the intrados. The solution to this problem is thus in laying down the cables used with a longer length (overlength) so that they endure the elongation or stretching.
In order to do this, the cables can be disposed in waves (sinusoidally, as represented in the annexed FIG. 1, which shows the cables 5 disposed in waves over the inner pipe of a pipe-in-pipe 2, 3 and thermal insulation 4) or they can be wound in coils around the inner pipe. They can also be wound using another known method, that being the S/Z process.
An approximate solution whose object is to enable flexion of a single envelope pipe is proposed by WO 86/03362. In that application, the cable is disposed in a sinusoidal wave of very low amplitude and low period (frequency) in order to sustain elongation or compression when the pipe is flexed.
The solutions provided up to now have their drawbacks: increasing the length of the cable, increasing electrical resistance (increasing the power required for heating), increasing the costs of manufacturing (S/Z reeling machine, or helical or attaching the cables to the inner pipe by the use of adhesives, wave solution), difficulty in positioning the cable which runs the risk of detaching the inner pipe, reducing the heating system's effectiveness no matter what solution is chosen.
Another drawback is the problem bound to the thermal insulation and its installation in connection with the cables (the width of the grooves made to enable the disposition in wave form of the cables reduces the amount of insulator that can be placed in the annular space).